The present invention relates generally to an electronic ballast for powering a high-pressure discharge lamp. The present invention more particularly relates to an electronic ballast configured for quickly restarting a high-pressure lamp even under conditions where the lamp has been previously shut down after a short operating time.
Referring to FIG. 22, an example of a high-pressure lamp ballast as known in the art is shown. The ballast includes a rectifier circuit DB1 for rectifying a commercial power source AC. A step-up chopper circuit 1 steps up a rectified voltage of the rectifier circuit DB1. A step-down chopper circuit 2 steps down an output voltage of the step-up chopper circuit 1 and provides a desired output. An inverter circuit 3 converts a DC output of the step-down chopper circuit 2 into a rectangular wave AC voltage and applies it to a high-pressure discharge lamp La. A start voltage generation circuit 4 generates a high voltage as required for lamp startup and applies it to the high-pressure discharge lamp La. A control circuit 50 is provided for controlling the respective operations of the step-up chopper circuit 1, the step-down chopper circuit 2, the inverter circuit 3, and the start voltage generation circuit 4.
The step-up chopper circuit 1 includes a capacitor C1 connected between output terminals of the rectifier circuit DB1, a series circuit of an inductor L1 and a diode D1 coupled to a positive output terminal of the rectifier circuit DB1, a switching element Q1 connected in parallel with the capacitor C1 through the inductor L1, and a capacitor C2 that is connected in parallel with the switching element Q1 through the diode D1. The step-up chopper circuit 1 generates a voltage VC2 across capacitor C2 that is a rectified voltage stepped up by the switching element Q1 being driven on and off.
The step-down chopper circuit 2 includes a series circuit of a switching element Q2 and an inductor L2 that is coupled to a positive terminal of the step-up chopper circuit 1, a diode D2 that is connected in parallel with the capacitor C2 through the switching element Q2, and a capacitor C3 that is connected in parallel with the diode D2 through the inductor L2. The step-down chopper circuit 2 generates a voltage Vc3 across the capacitor C3 that is the output voltage of the step-up chopper circuit 1 stepped down by the switching element Q2 being driven on and off, and provides a desired output signal.
The inverter circuit 3 includes a series circuit of switching elements Q3, Q4 and a series circuit of switching elements Q5, Q6 that are connected in parallel to the capacitor C3. The connection mid points of the switching elements Q3, Q4 and the switching elements Q5, Q6 are used as output terminals. The inverter circuit 3 converts an output voltage Vc3 of the step-down chopper circuit 2 into a rectangular wave AC voltage Vo by the switching elements Q3, Q6 and the switching elements Q4, Q5 being alternately driven on and off, and provides the converted voltage as an output signal.
The control circuit 50 includes a step-up chopper control module 51, a step-down chopper control module 52, a lighting transition module 53, and an inverter control module 54.
The step-up chopper control module 51 includes a step-up output detection circuit 51a and a step-up chopper control circuit 51b, wherein the step-up output detection circuit 51a detects the output voltage Vc2 of the step-up chopper circuit 1, and feeds it back to the step-up chopper control circuit 51b. The step-up chopper control circuit 51b controls the output voltage Vc2 to a constant voltage by driving on and off the switching element Q1 based on the detected output voltage Vc2 of the step-up chopper circuit 1.
The step-down chopper control module 52 includes a step-down output detection circuit 52a and a step-down chopper control circuit 52b. The step-down output detection circuit 52a detects the output voltage Vc3 of the step-down chopper circuit 2, and feeds it back to the step-down chopper control circuit 52b. The step-down chopper control circuit 52b drives on and off the switching element Q2 based on the output voltage Vc3 of the step-down chopper circuit 2 that was detected, controls an output current of the step-down chopper circuit 2 to a predetermined current that depends on the output voltage Vc3, and supplies a desired power output to the high-pressure discharge lamp La.
Based on the output voltage Vc3 of the step-down chopper circuit 2, the lighting transition module 53 detects an intended startup or shutdown of the high-pressure discharge lamp La, and outputs the transition result to the step-down chopper control module 52.
The inverter control module 54 alternately drives on and off the switching elements Q3, Q6 and Q4, Q5 of the inverter circuit 3.
The start voltage generation circuit 4 is equipped with a pulse transformer T1. A secondary winding N2 of the pulse transformer T1 is serially connected to the high-pressure discharge lamp La between the output terminals of the inverter circuit 3. A series circuit made up of a parallel circuit of a primary winding N1 of the pulse transformer T1, a switching element Q10, and a resistor R1 and a capacitor C4 is connected in parallel with a series circuit of the high-pressure discharge lamp La and the secondary winding N2. The switching element Q10 is a voltage-responsive switching element that becomes conductive when the voltage across it exceeds a predetermined voltage. The charging voltage of the capacitor C4 is controlled by the resistor R1 upon switching off of the switching element Q10.
Below, an operation of this conventional example will be explained using waveform diagrams as shown in FIGS. 23(a) to (h). First, at the time the high-pressure discharge lamp La is shut down or extinguished, the step-down chopper circuit 2 outputs a high DC voltage compared to the time of turning on the high-pressure discharge lamp La in order to make the high-pressure discharge lamp La start satisfactorily. Then, as shown in FIGS. 23(a), (b), driving signals G1, G2 of the switching elements Q4, Q5 and the switching elements Q3, Q6 of the inverter circuit 3 turn on and off alternately, which makes the switching element Q3, Q6 and the switching elements Q4, Q5 turn on and off alternately. Therefore, an output of the inverter circuit 3 becomes the rectangular wave AC voltage Vo as shown in FIG. 23(c).
When the voltage Vc4 across the capacitor C4 of the start voltage generation circuit 4 is charged by the rectangular wave AC voltage Vo as shown in FIG. 23(d), a voltage Vq10 across the switching element Q10 shown in FIG. 23(e) rises, and when it exceeds the predetermined voltage, the switching element Q10 is subjected to breakdown and becomes conductive. In particular, an amplitude of the rectangular wave AC voltage Vo is substantially identical to the output voltage Vc3 of the step-down chopper circuit 2. Denoting the output voltage of the step-down chopper circuit 2 as Vc3, and the voltage across capacitor C4 as Vc4, a voltage across switching element Q10 at the time of stability of the rectangular wave AC voltage Vo (except at the time of polarity inversion) becomes |Vc3|−|Vc4|, which does not reach a breakdown voltage of the switching element Q10, and the switching element Q10 remains turned off. However, when the polarity of the rectangular wave AC voltage Vo is reversed, the voltage Vc4 does not change rapidly because of charging through the resistor R1, a voltage |Vc3|+|Vc4 | is applied to both ends of the switching element Q10, the breakdown voltage is reached, and the switching element Q10 turns on.
When the switching element Q10 becomes conductive, a pulsed current flows in the primary winding N1 of the pulse transformer T1 with the capacitors C3, C4 acting as the power source, and a high-pressure pulse Vp shown in FIG. 23(f) is generated in the secondary winding N2 of the pulse transformer T1.
A lamp voltage Via shown in FIG. 23(g) that is the rectangular wave AC voltage Vo output by the inverter circuit 3, with the high-pressure pulse of the start voltage generation circuit 4 superimposed thereon, is applied across the high-pressure discharge lamp La, and the lamp La starts at time T1. When the lighting transition module 53 detects startup of the high-pressure discharge lamp La in the step-down chopper control module 52, the step-down output detection circuit 52a detects the output voltage Vc3 of the step-down chopper circuit 2. The step-down chopper control circuit 52b controls the output current of the step-down chopper circuit 2 (corresponding to a lamp current Ila of FIG. 23(h)) to be the predetermined current that depends on the output voltage Vc3, supplies the high-pressure discharge lamp La with desired power, and in steady state ignites the high-pressure discharge lamp La.
Upon a lighting failure of the high-pressure discharge lamp La, the step-down chopper control circuit 52b detects the transition of the high-pressure discharge lamp La from the lighting transition module 53, determines that it was turned off, and performs the restart operation as shown in the flowchart of FIG. 24. First, the input power source is provided (S101), the high-pressure pulse Vp is generated by the start voltage generation circuit 4 (S102), and after startup, lighting control is performed for steady state operation of the high-pressure discharge lamp La (S103). Then the high-pressure discharge lamp La is monitored for a lighting failure based on the transition result of the lighting transition module 53 (S104). If the high-pressure discharge lamp La does not produce a lighting failure, the lighting control is continued, but when the high-pressure discharge lamp La produces a lighting failure, the high-pressure pulse Vp is generated again.
The inside of an arc tube gradually reaches a high temperature and a high pressure from the time of the startup, and the high-pressure discharge lamp La eventually reaches a stable lighting state. If the high-pressure discharge lamp La turns off from the stable lighting state, since the inside of the arc tube has reached a very high temperature and high pressure, it is known that the breakdown voltage required for discharge in the arc tube rises abruptly in a short time from about 100 V at the time of stable lighting, reaching 10 kV or more. In order to restart the high-pressure discharge lamp La immediately after turning it off from the stable lighting state, it is therefore necessary to apply a startup voltage of a 10-30 kV to the high-pressure discharge lamp La. However, it is difficult to secure insulation performance of the main body and wiring of the lamp fixture, and if q 30 kV start voltage is applied it may exceed insulation performance of the lamp fixture due to a structural problem of the high-pressure discharge lamp La. For example, in the case of a typical high-pressure discharge lamp La in which an outer tube 102 is attached to a base 101 of an Edison base, as shown in FIG. 25, and an arc tube 103 is housed in the outer tube 102, if the start voltage of 10 kV or more is applied there is a fear that dielectric breakdown may occur at the base 101 and a required voltage may not be applied to the inside of the arc tube 103.
Therefore, a typical high pressure ballast is designed so as to generate the breakdown voltage required for discharge inside the arc tube (e.g., about 4 kV) as a starting high voltage in a state where the target high-pressure discharge lamp La is sufficiently cooled. Therefore, at a time immediately after the high-pressure discharge lamp La is extinguished from a stable lighting state, since the breakdown voltage required for discharge inside the arc tube is high, it is impossible to re-light it. Then, after a period of time from the turning off of the lamp has elapsed, and thus the temperature inside the arc tube decreases and the breakdown voltage required for discharge inside the arc tube falls below the starting high voltage that can be generated by the high-pressure lamp ballast, a restart again becomes possible.
Depending on the type of high-pressure discharge lamp La, the structure of the lamp fixture, and the particulars of its installation, it is understood that this restart may require approximately ten minutes. During that time, the high pressure ballast keeps generating the starting high voltage continuously or intermittently. The state of the high-pressure discharge lamp La immediately after turning off, where the inside of the arc tube is at a high temperature and a high pressure, and the breakdown voltage is very high, is substantially the same state of an unloaded condition where the high-pressure discharge lamp La is not connected. Because it is not easy to differentiate a state of the high-pressure discharge lamp La immediately after being turned off from an unloaded condition, the ballast is configured to generate the starting high voltage even in the unloaded condition as described above.
Moreover, even in the case where the high-pressure discharge lamp La approaches the end of its life and becomes impossible to start, the high-pressure lamp ballast continues to generate the starting high voltage.
Referring now to FIG. 26, another high-pressure lamp ballast configuration of the prior art is shown to address the situation where the high pressure ballast generates the starting high voltage even in an unloaded condition or in an end-of-life state of the high-pressure discharge lamp La. This high-pressure lamp ballast has a power supply module 201 for supplying electric power to the high-pressure discharge lamp La, a starting high-voltage generating module 202 for generating the starting high voltage, a current detection module 203 for detecting a current supplied to the high-pressure discharge lamp La, and a monitoring control module 204 that monitors a state of the high-pressure discharge lamp La based on the lamp current detected by the current detection module 203 and controls the power supply module 201 and the starting high-voltage generating module 202.
When a light switch (not shown) is turned on to trigger startup (S201), as shown in a flow chart of FIG. 27, the monitoring control module 204 resets a counter (not shown) that is provided therein to “0” (S202). Subsequently, the monitoring control module 204 applies the starting high voltage to the high-pressure discharge lamp La by driving the starting high-voltage generating module 202 and increments the counter value (S203). Then, it determines whether the lamp current is more than or equal to a predetermined value (S204), and if so the startup is determined to have succeeded and the start control is ended (S205). On the other hand, if the lamp current is less than the predetermined value, it is determined whether the count value is equal to a predetermined number of times “4” (S206). If the count value is less than “4”, the process returns to Step 203 where the starting high voltage is applied again. If the count value is “4”, application of the starting high voltage has already been repeated four times consecutively, and it is determined that the startup failed. Operation of the starting high-voltage generating module 202 is halted, power flow from the power supply module 201 is halted (S207), and the start control is ended (S205). Thus, at the time of start failure, risks of damage to the high-pressure lamp ballast and electric shock from application of the starting high voltage for a long period of time are reduced.
Another example of a high-pressure lamp ballast, as shown in FIG. 28, prevents discharge inside the outer tube (discharge at a location 500 between the lead wires 104, 105 that are connected to both ends of the arc tube 103 as shown in FIG. 25) when the starting high voltage is applied to the high-pressure discharge lamp La in a state where the breakdown voltage has risen immediately after the lamp is extinguished from the stable lighting state. In this high-pressure lamp ballast, when the AC power source AC is provided, a power source control circuit 310 operates to make a control circuit 309 send control signals to a step-up inverter 303, a step-down inverter 304, a rectangular wave circuit 306, and a start pulse generation circuit 307, respectively, each of which then starts its operation. The step-up inverter 303 steps up an output rectified by a rectifier circuit 302 to a specified voltage; the step-down inverter 304 regulates the output so that a current flowing into the high-pressure discharge lamp La may become a specified current. The square wave circuit 306 outputs an AC rectangular wave voltage of a specified frequency to the high-pressure discharge lamp La. A start pulse generation circuit 307 generates high-pressure pulse and starts the high-pressure discharge lamp La. In addition, a current sensing resistor 305 detects a current of the high-pressure discharge lamp La and lighting transitions may be detected.
When the high-pressure discharge lamp La has a lighting failure, the control circuit 309 controls the start pulse generation circuit 307 to perform a restart. The operation is such that, as shown in the flowchart of FIG. 29, first a lighting switch (not shown) turns on to trigger the power supply (S301). Then the start pulse generation circuit 307 generates the starting high voltage as pulses with a predetermined frequency and applies it to the high-pressure discharge lamp La (S302). Next, after resetting the lighting time counter for counting the operating time of the high-pressure discharge lamp La (S303), the intensity of the high-pressure discharge lamp La is controlled by the AC power from the square wave circuit 306 (S304), and the operating time of the high-pressure discharge lamp La is recorded by the operating time counter (S305).
When a lighting failure of the high-pressure discharge lamp La in the lighting state is monitored (S306), and the lamp is turned off, the operating time counter of the high-pressure discharge lamp La determines whether the operating time exceeded a predetermined time (e.g., 10 minutes) (S307). When the operating time counter does not exceed the predetermined time, the process returns to the start control of S302.
When the operating time counter of the high-pressure discharge lamp La exceeds the predetermined time, the count number k is reset (S308), the count number k is counted up only by unity, operation is halted for a time period that permits the arc tube to restart easily (S309), and another restart by the start pulse generation circuit 307 is performed (S310). Next, the lighting transition of the high-pressure discharge lamp La is monitored (S311), and if it lights normally the process returns to Step S304. If it does not light normally, it is determined whether the number of discontinuous arc discharges exceeded, for example, a predetermined number of times (e.g., 1024 times) (Step S312). If it did not exceed the predetermined number of times, the process will return to Step S310. If it exceeds the number, it is judged whether the count number k is 3 (S313), and then if the count number k is less than 3, the process returns to Step S309. If the count number k is 3, the high-pressure lamp ballast will terminate operation for protection purposes (S314).
Therefore, in a period where the restart is impossible (with the starting high voltage generated by the high-pressure lamp ballast when the high-pressure discharge lamp becomes stable), the breakdown voltage rises, and subsequently is turned off, the restart is not performed. Application of a needless starting high voltage is suppressed and inside-outer-tube discharge and electric shock are prevented.
In yet another example of a high-pressure lamp ballast as shown in FIG. 30, the high-pressure discharge lamp La is connected to the AC power source AC through a power source switch (not shown) and a stabilizer 401. Moreover, a start pulse generation circuit 402 is connected to the stabilizer 401, and at the time of startup the start pulse generation circuit 402 operates to make the stabilizer 401 apply the starting high voltage to the high-pressure discharge lamp La, which causes a starting operation of the high-pressure discharge lamp La. Then, when a lighting detection circuit 403 detects that the high-pressure discharge lamp La lights and the lamp current flows, operation of the start pulse generation circuit 402 is prohibited and generation of the starting high voltage is restricted.
There is further provided a pulse control circuit 404 for restricting the generation of the starting high voltage when an attempt is made to again input power and perform the restart, after the lamp La passed through the continuous lighting state and subsequently the power source was temporarily cut off. This pulse control circuit 404 is provided with a timer circuit 404a that is based on a counter etc. and is for counting the lighting time of the lamp La up to the shutdown by power source interruption. An output side of this timer circuit 404a is connected with a conversion circuit 404b. This conversion circuit 404b converts the lighting time that was counted by the timer circuit 404a into a required pulse delay time Tw according to a constant predetermined conversion ratio. In addition, a timer circuit 404c that counts an elapsed time Tp from the shutdown by power source interruption to return of the power source is provided. Outputs of these conversion circuit 404b and timer circuit 404c are input into a comparator 404, and its comparison result is output to an output circuit 404e. 
The output circuit 404e controls a pulse generating operation of the start pulse generation circuit 402 in response to a comparison output of the comparator 404. If Tw≦Tp after a comparison of a pulse waiting time Tw and the elapsed time Tp, it will generate the high-pressure pulse. This is done by making a light-emitting diode 405a emit instantaneously, making a photo-thyristor 405b that is photo-coupled thereto conductive, gate-triggering a bidirectional thyristor 406 that is serially connected to the start pulse generation circuit 402, and making the start pulse generation circuit 402 operate. However, if Tw>Tp as the result of the comparison, after a time (Tw-Tp) elapses a high-pressure pulse is generated by making the light-emitting diode 405a emit light, making the photo-thyristor 405b that is photo-coupled thereto conductive, gate-triggering the bidirectional thyristor 406, and making the start pulse generation circuit 402 operate.
In this way, the control circuit self-determines a first time required from input of the power source until the high-pressure discharge lamp La may become restarted, decides a second time until the lamp is to be restarted after having been turned off according to the determination, and generates the starting high voltage after the appropriate time elapsed, whereby application of a needless starting high voltage is suppressed.
In typical high-pressure lamp ballasts such as those described above, when they are turned off before reaching a stable lighting state, the inside of the high-pressure discharge lamp does not reach a high temperature or a high pressure, and it is considered that a rise of the breakdown voltage required for discharge is also relatively small immediately after turning off. Therefore, the ballast attempts to start the lamp by applying the starting high voltage immediately.
However, depending on the type of high-pressure discharge lamp, it is hard to start even when the restart is performed immediately after the lamp is turned off before reaching a stable lighting mode. This phenomenon occurs particularly often in a high pressure discharge lamp that contains a metal iodide. This is due primarily to the starting high voltage making the amalgam (being a mercury compound) and the iodide scatter and adhere onto a tube wall near an electrode, wherein an easy-to-discharge path is formed along a tube wall and, upon a restart, discharge takes place along this discharge path.
Usually, as the inside of the arc tube achieves a high temperature and a high pressure due to a continuing discharge after startup, amalgam and an iodide that adhere to the tube wall evaporate. However, if the lamp turns off before these materials evaporate, the inside of the arc tube of the high-pressure discharge lamp La remains in an unstable state, and tends to only gradually return to a stable initial state. If the starting high voltage is applied in a state where the amalgam and iodide adhere to the tube wall in this way, a mode where glow discharge does not shift to main discharge continues because of occurrence of discharge along the above-mentioned discharge path formed on the tube wall. Even when the startup is attained, the iodide consumes electrons to be used for discharge, whereby discharge time is extended and the start time becomes prolonged. Moreover, in the high-pressure lamp ballast that halts its operations due to continuation of abnormal discharge, there arises a problem that the high pressure discharge lamp will not light.
FIG. 31 shows results of an experiment to determine whether restart is possible in the case where a high color-rendering and high-efficiency metal halide lamp is made to operate for a short time and subsequently is turned off, and after a predetermined time elapsed after being turning off, the power source is input again. This experimental result shows that in the case where the operating time is about 10 to 30 seconds, there is a domain B1 in which the restart fails depending on the length of off time, or delay, after the operating time.